A capacitance microphone converts acoustic signal to its mechanical vibration, and further converts to a capacitance signal. FIG. 1A is a schematic diagram of parallel capacitive microphone in the prior art. Two thin layers 101 and 102 are placed closely in almost parallel. One of them is fixed backplate 101, and the other one is movable/deflectable membrane/diaphragm 102, which can be moved or driven by sound pressure. Diaphragm 102 acts as one plate of a capacitor, and the vibrations thereof produce changes in the distance between two layers 101 and 102, and changes in the mutual capacitance therebetween.
“Squeeze film” and “squeezed film” refer to a type of hydraulic or pneumatic damper for damping vibratory motion of a moving component with respect to a fixed component. Squeezed film damping occurs when the moving component is moving perpendicular and in close proximity to the surface of the fixed component (e.g., between approximately 2 and 50 micrometers). The squeezed film effect results from compressing and expanding the fluid (e.g., a gas or liquid) trapped in the space between the moving plate and the solid surface. The fluid has a high resistance, and damps the motion of the moving component as the fluid flows through the space between the moving plate and the solid surface.
In capacitive microphones as shown in FIG. 1A, squeeze film damping occurs when two layers 101 and 102 are in close proximity to each other with air disposed between them. The layers 101 and 102 are positioned so close together (e.g. within 5 μm) that air can be “squeezed” and “stretched” to slow movement of membrane/diaphragm 102. As the gap between layers 101 and 102 shrinks, air must flow out of that region. The flow viscosity of air, therefore, gives rise to a force that resists the motion of moving membrane/diaphragm 101. Squeeze film damping is significant when membrane/diaphragm 101 has a large surface area to gap length ratio. Such squeeze film damping between the two layers 101 and 102 becomes a mechanical noise source, which is the dominating factor among all noise sources in the entire microphone structure.
Co-pending U.S. application Ser. No. 15/393,831 to the same assignee, which is incorporated herein by reference, teaches a so-called lateral mode microphone in which the movable membrane/diaphragm does not move into the fixed backplate, and the squeeze film damping is substantially avoided. An embodiment of the lateral mode microphone is shown in FIG. 1B. First electrical conductor 201 is stationary, and has a function similar to the fixed backplate in the prior art. A large flat area of second electrical conductor 202, similar to movable/deflectable membrane/diaphragm 102 in FIG. 1A, receives acoustic pressure and moves up and down along the primary direction, which is perpendicular to the flat area. However, conductors 201 and 202 are configured in a side-by-side spatial relationship, not one above another. As one “plate” of the capacitor, conductor 202 does not move toward and from conductor 201. Instead, conductor 202 laterally moves over, or “glides” over, conductor 201, producing changes in the overlapped area between 201 and 202, and therefore varying the mutual capacitance therebetween. A capacitive microphone based on such a relative movement between conductors 201 and 202 is called lateral mode capacitive microphone.
However, such a lateral mode capacitive microphone suffers a problem. An acceleration of the microphone may affect the accuracy of sound detection. An acceleration of 1G on the direction that is normal to the flat area of conductor 202 (or membrane 202) causes a signal to be detected, whose value may be 13% of 1 Pa sound pressure. Signal to Acceleration Ratio (SAR) may be used to define this effect. For example, the SAR for a single slot design structure disclosed in the co-pending U.S. application Ser. No. 15/393,831 can be around 7.6, which is much smaller than the typical SAR 70-100 for a conventional MEMS microphone. A microphone with low SAR will suffer from inaccurate signal detection when the microphone vibrates at low frequency. For example, if the microphone, or a device using such a microphone (e.g. a cellphone), is being used in a running automobile, the shake or vibration of the device along the automobile is actually an acceleration applied on membrane 202 and may be “misread” as a sound signal.
Co-pending U.S. application Ser. No. 15/623,339 to the same assignee, which is incorporated herein by reference, provides an improved lateral mode capacitive microphone, in which the low SAR effect is compensated. Co-pending U.S. application Ser. No. 15/623,339 teaches a motional sensor is used in the microphone to estimate the noise introduced from acceleration or vibration of the microphone for the purpose of compensating the microphone output through a signal subtraction operation. In an embodiment, the motional sensor is identical to the lateral microphone, except that the movable membrane in the motional sensor has air ventilation holes for lowering the movable membrane's air resistance, and making the movable membrane responsive only to acceleration or vibration of the microphone.
However, random air ventilation holes will not have the most desired SAR compensation. Advantageously, the present invention provides a solution to such a problem.